An important issue in 3D-Multiple Input Multiple Output (3D-MIMO) is CSI measurement and feedback. At present, there mainly are two schemes as follows.
The first scheme is a beamformed CSI-RS-based measurement and feedback scheme. Specifically, as shown in FIG. 1, in the present scheme, it is essentially necessary for an evolved Node B (eNB) to configure a User Equipment (UE) with one or more UE-specific beamformed CSI-RS resources, herein a UE-specific beamformed CSI-RS resource may be a Non-Zero Power (NZP) CSI-RS resource, or may be a CSI-RS port, or may be a CSI process, or may be a Discovery Reference Signal (DRS), or the like.
The second scheme is a non-precoded CSI-RS-based measurement and feedback scheme. Specifically, as shown in FIG. 2, in the present scheme, a CSI-RS sent by an eNB to each UE of a cell covered by the eNB is a cell-specific non-precoded CSI-RS, that is, CSI-RSs measured by an indefinite number of UEs in the cell covered by the eNB are the same, and depend on a port number.
Besides, the second scheme usually needs to design a complex 16/32/64 port codebook, and the first scheme does not necessarily need to design a 16/32/64 port codebook, may be implemented by channel reciprocity and is particularly applied to a Time-Division Duplex (TDD) system, so that the first scheme is superior to the second scheme in terms of a codebook resource, and furthermore, the first scheme is superior to the second scheme in terms of CSI-RS coverage performance.
However, in terms of CSI-RS overheads, the second scheme usually adopts a cell-specific non-precoded CSI-RS, that is, CSI-RSs measured by an indefinite number of UEs in a cell are the same, and depend on a port number. The first scheme adopts a UE-specific beamformed CSI-RS, that is, CSI-RSs measured by different UEs are different, such that in the presence of a small number of UEs, CSI-RS overheads of the first scheme are lower than those of the second scheme. However, as UEs are increasing, the number of CSI-RSs needed by the first scheme will be increased in proportion. When the number of the UEs is increased to a certain extent, the CSI-RS overheads of the first scheme will be much higher than those of the second scheme.
As a result, a current technical problem to be urgently solved is how to reduce CSI-RS overheads of the first scheme, particularly, CSI-RS overheads of the first scheme in the presence of a large number of UEs so as to maintain the advantages of the first scheme as usual.